Fix performance issue incurred by removing monitor thread.

When the monitor thread is used, most threads in the team directly go to
sleep if the copy of bt_intervals/bt_set is not available in the cache,
and this happens at least once per thread in the wait function, making the
overall performance slightly better.
This change tries to mimic this behavior by using the bt_intervals cache,
which simply keeps the blocktime interval in terms of the platform-dependent
ticks or nanoseconds.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D28906

llvm-svn: 293312